The invention to be described hereinafter is closely related to an invention made by the present inventor in collaboration with Messrs. J.ENGDAHL and R. HUGUENIN and set forth in patent application ser. no. 650,643 (OM 355). In the earlier application there was given a solution to the problem of providing a high frequency high precision quartz resonator suitable for use in time keeping instruments such as wrist watches. The solution proposed was contrary to all known solutions for the provision of high frequency quartz resonators and indeed solutions which have been used over the past 35 years. In so proposing these solutions the inventors of the prior application made use of what may be referred to as coupled vibration modes.
The present invention seeks to extend the teachings set forth in the earlier filed application to multiple resonators or filters.
The utilization of a bar of quartz or other piezoelectric materials having thereon several energy trapping zones in order to obtain filter action is well known. The following publications give certain theoretic aspects and practical realizations of such filters:
1. M.Onoe and H.Jumonji Analysis of piezoelectric resonators vibrating in trapped energy modes., Electr. & Comm. Eng. Japon Vol. 48, pg. 84(1965) PA1 2. R.A.Sykes, W.L.Smith and W.J.Spencer Monolithic crystal filters, IEEE Intern. Convention Record, part. III, pg. 78,(1967) PA1 3. H.Mailer and D.R.Beuerle Incorporation of multi-resonator crystals into filters for quantity production Proc 20th annual Symposium on frequency control, pg. 309(1966) PA1 1.1 to 1.9 for coupling with the 2nd flexure harmonic PA1 2.2. to 3.6 for coupling with the 4th flexure harmonic PA1 3.6 to 5.3 for coupling with the 6th flexure harmonic PA1 5.1 to 6.8 for coupling with the 8th flexure harmonic PA1 6.5 to 8.4 for coupling with the 10th flexure harmonic PA1 8 to 10.4 for coupling with the 12th flexure harmonic
These publications in each case analyse a situation in which thickness shear vibrations occur and wherein the electrodes occupy only a small portion of the surface of the piezoelectric bar or plate. Such a restriction is required in view of the following phenomena:
1. In a thin plate of finite dimensions a thickness shear mode is always coupled to a flexure mode which propagates in the same direction as that of the displacement of material particles due to the thickness shear vibrations. For example in a quartz bar or plate in the AT cut the thickness shear mode TS.sub.1 is always coupled to a flexure mode F.sub.1. The mode TS.sub.1 takes place in a plane XY' with a displacement of the particles of the material in the X direction of the quartz (electrical axis). The mode F.sub.1 takes place in the same plane XY' and propagates along the X axis of the quartz.
Such coupling gains in importance to the extent that the ratio a/b becomes smaller, a being the lateral dimension of the quartz bar in the direction X and b the thickness of the plate.
The influence of this coupling shows itself in particular on the frequency spectrum where when the ratio a/b becomes small the frequency no longer depends uniquely on the thickness b of the plate, but as well to an important extent on the lateral dimension a.
Thus, should one wish to utilize a pure thickness shear mode, to simplify the theoretical analysis and eventual manufacture in order that the resonance frequency should depend only from the thickness of the plate it is necessary that the ratio a/b have a large value; in practice a ratio a/b equal to or greater than 30 is used. Also in the example of mass production (reference 3., hereinabove, pg. 322, FIG. 3) there is shown a filter for a frequency of 5.3 MHz which corresponds to a thickness of 0.31 mm and where the dimension along the X axis equals 0.46 inches,(11.7 mm) thus giving a ratio a/b = 38.
2. An almost pure thickness shear vibration results in only an amplitude attenuation outside the energy trapping zone. Thus a considerable distance is necessary between the edge of the energy trapping zone (for example the edge of the electrode) and the edge of the quartz in order to avoid absorptions or reflexions which may lower the quality of the filter.
In practice this type of filter is used for frequencies higher than 3-4 MHz. Below this limit the plates become excessively large.
For frequencies lower than 1 MHz modes of vibrations other than thickness shear are utilized. For example the British Pat. no. 1,361,622 uses a width extension mode.
With this latter mode there may be obtained filters having reasonable dimensions for frequencies down to approximately 262 kHz. Such mode, however, is no longer utilizable for for frequencies above 1 MHz unless one utilizes harmonics which will probably result in an excessively great impedance. Another inconvenience of a longitudinal mode in quartz is its unfavourable thermal behavior compared to that of a thickness shear mode in the AT cut.
It will of course be realized that although the discussion thus far has been in consideration of the particular qualities of quartz for use in resonators, like principles may be applied to other materials particularly where other applications are sought, as for instance in the design of filters. Where other materials are employed it will be obvious that the various crystal structures employed will probably vary considerably from quartz and thus the method of cutting as well as the elastic constants to be applied will be different. Nevertheless the principles taught herein are equally valid, no matter what piezoelectric material might be employed for a particular filter operating in this fashion.
The invention thus seeks to realize a multiple resonator or filter obtained from a bar of piezoelectric material the contour of which has a rectangular form and shows at least two energy trapping zones. Such a bar or plate should have extremely reduced dimensions. Thus relative to known practical realizations it is possible to reduce by a factor of 10 the dimension in the X direction of a filter cut out of a quartz plate. The choice of dimensional ratios and of the orientation enables a very close control over the coupling of the different energy trapping zones as well as control over the thermal and electrical characteristics. Such principles are applicable to various piezoelectric materials.
Such filters will find use wherever the available space becomes critical in making the choice.